(a) Field of the Invention
The present invention relates to a polymer electrolyte membrane for a fuel cell, and a membrane-electrode assembly and a fuel cell system including the same.
(b) Description of the Related Art
A fuel cell is a power generation system for producing electrical energy through an electrochemical redox reaction of an oxidant and hydrogen in a hydrocarbon-based material such as methanol, ethanol, or natural gas.
Such a fuel cell is a clean energy source that can replace fossil fuels. It includes a stack composed of unit cells, and produces various ranges of power output. Since it has a four to ten times higher energy density than a small lithium battery, it has been highlighted as a small portable power source.
Representative exemplary fuel cells include a polymer electrolyte membrane fuel cell (PEMFC) and a direct oxidation fuel cell (DOFC). The direct oxidation fuel cell includes a direct methanol fuel cell that uses methanol as a fuel.
The polymer electrolyte membrane fuel cell has an advantage of high energy density and high power, but it also has problems in the need to carefully handle hydrogen gas and the requirement for additional devices such as a fuel reforming processor for reforming methane or methanol, natural gas, and the like in order to produce hydrogen as the fuel gas.
On the contrary, a direct oxidation fuel cell has lower energy density than that of the polymer electrolyte fuel cell, but has the advantages of easy handling of the polymer electrolyte membrane fuel cell, a low operation temperature, and no need for additional fuel reforming processors.
In the above-mentioned fuel cell system, a stack that generates electricity substantially includes several to scores of unit cells stacked adjacent to one another, and each unit cell is formed of a membrane-electrode assembly (MEA) and a separator (also referred to as a bipolar plate). The membrane-electrode assembly is composed of an anode (also referred to as a “fuel electrode” or an “oxidation electrode”) and a cathode (also referred to as an “air electrode” or a “reduction electrode”) that are separated by a polymer electrolyte membrane.
A fuel is supplied to an anode and adsorbed on catalysts of the anode, and the fuel is oxidized to produce protons and electrons. The electrons are transferred into a cathode via an external circuit, and the protons are transferred into the cathode through the polymer electrolyte membrane. In addition, an oxidant is supplied to the cathode, and then the oxidant, protons, and electrons are reacted on catalysts of the cathode to produce electricity along with water.